This invention relates to the field of electro-optic switching and modulating devices and more particularly to electro-optic switching and modulating devices comprised of nonlinear optical polymer material.
As optical data processing circuits approach multigigahertz operation rates, the need arises for optical signal transmission for multichip module-to-multichip module interconnection on a common board and for board-to-board interconnection through a common backplane. Currently employed electrical interconnects become impractical at multigigahertz operating rates due to electromagnetic interference and excessive power loss. As electrical interconnects are replaced with optical interconnects there will be a need for transverse electro-optic devices for signal routing control and signal modulation for high data rate optical signals. For purposes of this invention, high data rate optical signals are defined as those exceeding 10 gigabits per second. Nonlinear optical polymer transverse electro-optic devices have several attractive characteristics which many researchers have tried to capitalize on in the past decade in an effort to realize electro-optic modulators and switches for multichip module-to-multichip module and board-to-board optical interconnects for high data rate optical signals. Nonlinear optical polymers have the property for which a polarization and, hence, a birefringence can be induced by an applied electric field and includes organic-based materials, inorganic-based materials and, ceramic materials, as well as combinations and mixtures thereof.
A top view of a transverse electro-optic directional coupler switch is represented in the drawing of FIG. 1. As the basic form of a 2-input.times.2-output optical switch, the directional coupler, shown in FIGS. 1a and 1b is known, with FIG. 1a showing a directional coupler without applied switching voltage and FIG. 1b showing a directional coupler with applied switching voltage. A directional coupler type of electro-optic switch is one which controls transfer of an optical signal from one channel waveguide to another by both voltage independent and voltage dependent phase changes. A "transverse" electro-optic controlling device refers to an electro-optic device where the optical signal is traveling normal to the applied electric field or parallel to the electrodes. The applied voltage causes a change in the dielectric properties of the material and hence renders a change in the index of refraction of the material in the switch's coupling portion which introduces a .pi./2 phase change through an electro-optic effect. The FIG. 1a top view illustrates a directional coupler having ridge type waveguides etched in a layer of nonlinear optical polymer material and passive polymer material.
Parallel channel waveguides separated by a finite distance for receiving an optical signal are represented in both FIGS. 1a and 1b at 101, 102 and 112 and 113, respectively. A single optical input signal is considered for purposes of the present discussion; this signal is represented by the bold arc at 103 and 114 in FIGS. 1a and 1b, respectively. A symmetric mode component of this optical signal, as represented at 105, and an antisymmetric mode of the optical signal as represented at 104 in FIG. 1a and at 116 and 115 in FIG. 1b, respectively, is generated upon the optical signal entering the directional coupler because of the mode coupling properties of two adjacent optical waveguides with small separation and these modes travel along the length of the channel or switch, over such lengths as are represented at 106 and 121 in FIGS. 1a and 1b respectively. The phase of the two modes shift as the respective signals travel the length of the waveguides, as is represented in the dotted, curving lines, shown at 108 in FIG. 1a and at 119 in FIG. 1b and the solid, curving lines shown at 107 and 120 in FIGS. 1a and 1b respectively. The symmetric mode is the mode of propagation within the waveguide region in which the optical signal is launched and the antisymmetric mode, the mode of propagation within the other waveguide region. With no voltage applied to the FIG. 1 switches, complete transfer of light from one channel to the next occurs at a distance that introduces a voltage independent .pi./2 phase shift to the modes so that one mode couples completely to the other. Complete mode coupling and light transfer occurs at the output waveguides at 126 in FIG. 1a and thereafter the complete optical signal at 111 exits the waveguide at 128 in FIG. 1a.
Applying an electric field to one of the channels of the directional coupler of FIG. 1b over the distance L represented at 121 from the voltage source shown at 122 in FIG. 1b will alter the dielectric properties of the coupler's nonlinear material subjected to the electric field, hence changing the index of refraction of the material and introducing a voltage dependent .pi./2 phase shift in the signal modes 115 and 116 and thereby switching the waveguide through which the optical signal exits from 129 to 130 as represented at 125 in FIG. 1b.
Similarly, electro-optic modulation is a rotation of the polarization of the optical signal due to an applied electric field. Maximum modulation occurs at an electric field that induces a .pi./2 polarization rotation.
Past research has focused on exploiting the electro-optic properties of nonlinear optical polymers with optimized optical, structural and mechanical properties in an attempt to achieve high performance nonlinear optical polymer transverse electro-optic devices, such as switches and modulators. Nonlinear optical polymers have several attractive potential characteristics on which many researchers have tried to capitalize over the past decade. These include a high electro-optic coefficient enabling low voltage operation, a low dielectric constant for high speed modulation, low temperature processing enabling integration of optics with electronics, excellent refractive index match with optical fiber materials and simplified fabrication for lower cost. A prior art conventional nonlinear optical polymer transverse electro-optic device is shown in cross-section in FIG. 2. FIG. 2 illustrates a nonlinear optical polymer core layer at 201; the optical signal is transmitted through waveguides. Passive polymer cladding layers are located at 202 and 203 in FIG. 2 and these layers operate to confine the optical signal within the core layer and limit propagation losses. Metal layers, or electrodes, initially used for poling the FIG. 2 device and during operation used for providing switching voltage are shown at 204 and 205 in FIG. 2. A voltage applied to the upper electrode 204 produces an electric field between the upper and lower electrodes, across the core polymer layer at 201 and hence changes the dielectric properties of the material, which in turn renders a change in the refractive index of the material. This is an electro-optic effect that produces a voltage dependent .pi./2 phase shift in the layer 201 communicated signal modes, causing the optical signal to switch from one waveguide to the next in the layer 201 of the FIG. 2 structure.
Several technical barriers have heretofore prevented the use of nonlinear electro-optic polymers from progressing toward commercialization much further than research devices. One of the barriers is the poling voltage requirements of such polymers. In order to align the molecules in the nonlinear optical polymer core layer 201 in FIG. 2, for example, the polymer must be "poled" once prior to the initial operation; i.e., the polymer material must be heated and subjected to a high voltage to secure the desired nonlinear optical properties of the material. The polymer material may need poling at other times during the life of the device in the event the design parameters of the device have been exceeded. For example, if the device is exposed to a temperature outside the its design parameters, the nonlinear characteristics of the polymer core layer will be lost and the material will have to be poled again. A conventional nonlinear optical polymer transverse electro-optic device with three layers of polymer material between electrodes--two cladding layers and a core layer--totaling six to eight micrometers of thickness, for example, results in a poling voltage requirement of 900 to 1200 volts (150 volts per micron of polymer thickness). Voltage levels of these magnitudes prevent easy integration of these electro-optic devices with electronics because the poling of any such electro-optic device fabricated on a single chip with other electrical devices would effectively cause high voltage damage to the other electronic and electro-optic circuit devices at the time the polymer was poled. The electro-optic device, therefore, cannot be poled insitu within an electronic integrated circuit and must be poled external to the electronic circuit. This makes monolithic integration within integrated circuits impractical. The impracticality stems from the fact that the device is fabricated and poled separately from the electronic circuit on another substrate. To interface with the electronic circuit, the conventional electro-optic device must therefore be properly aligned with the other chip components and glued in place. These steps add to the complexity of manufacturing and are much less fabrication tolerant; moreover, the poling operation may be difficult if not impossible to perform at a later time during the operating life of the device if the polymer loses its nonlinear properties.
Another barrier that has prevented nonlinear optical polymer transverse electro-optic devices from progressing much past the research stage is the required device length. Conventional switching devices are typically 14 to 27 millimeters in length. Such a length is required in a conventional nonlinear optical polymer transverse electro-optic device, for example, to enable use at a reasonable switching voltage. However, such a length also prevents integration of the device into integrated electronic circuits or electronic multichip modules.
The present invention overcomes the barriers to commercial use of nonlinear electro-optic polymers for use in fabricating electro-optic devices. Using the method and device of the present invention, it is feasible to have an array of these switches in an integrated circuit chip small enough to place within a multichip module. Also, monolithic integration with electronic circuits as well as insitu poling are possible.